With the development of computer technologies and the improvement of image recognition technologies, data identification utilizing media such as bar codes has been widely used for identification of product data and the like. It is expected that the amount of data to be identified will be further increased in the future. On the other hand, data identification utilizing bar codes and the like is disadvantageous in that a bar code reader is required to be in contact with bar codes, and the amount of data stored in bar codes cannot be increased much. Therefore, non-contact data identification and an increase in the storage capacity of media are demanded.
In view of the foregoing, an ID chip using an IC has been developed in recent years. The ID chip stores required data in a memory circuit of an IC chip and the data is read by a non-contact means, generally by a wireless means. It is expected that practical application of such an ID chip will simplify commercial distribution and the like and reduce the cost while maintaining high security.
An identification system using an ID chip is briefly described with reference to FIG. 4. FIG. 4 is a schematic view showing an identification system for obtaining identification data of a bag without contact. An ID chip 401 storing specific identification data is attached to or incorporated in a bag 404. Electromagnetic waves are transmitted from an antenna unit 402 of an interrogator (also called a reader/writer) 403 to the ID chip 401. Receiving the electromagnetic waves, the ID chip 401 sends its identification data back to the antenna unit 402. The antenna unit 402 transmits the received identification data to the interrogator 403, and the interrogator 403 determines the identification data. In this manner, the interrogator 403 can obtain the data of the bag 404. Such a system enables distribution management, calculation, prevention of counterfeit goods, and the like.
The ID chip has, for example, a configuration shown in FIG. 2. A semiconductor device 200 used as an ID chip includes an antenna circuit 201, a rectifying circuit 202, a stabilizing power source circuit 203, a modulation circuit 204, an amplifier 205, a logic circuit 206, a demodulation circuit 207, an amplifier 208, a logic circuit 209, a memory control circuit 210, and a memory circuit 211. The antenna circuit 201 includes an antenna coil 301 and a tuning capacitor 302 (FIG. 3A). The rectifying circuit 202 includes diodes 303 and 304, and a smoothing capacitor 305 (FIG. 3B).
The operation of such an ID chip is described hereinafter. An AC signal received by the antenna circuit 201 is half-wave rectified by the diodes 303 and 304, and then smoothed by the smoothing capacitor 305. The smoothed voltage which has many ripples is stabilized by the stabilizing power source circuit 203, and the stabilized voltage is supplied to the modulation circuit 204, the amplifier 205, the logic circuit 206, the demodulation circuit 207, the amplifier 208, the logic circuit 209, the memory control circuit 210, and the memory circuit 211. Meanwhile, a signal received by the antenna circuit 201 is inputted as a clock signal to the logic circuit 209 through the amplifier 208. A signal inputted from the antenna circuit 201 is demodulated by the demodulation circuit 207 and inputted as data to the logic circuit 209.
The data inputted to the logic circuit 209 is decoded. The interrogator encodes data by deformable mirror code, NRZ-L code or the like, and the logic circuit 209 decodes the data. The decoded data is transmitted to the memory control circuit 210, thereby data stored in the memory circuit 211 is read. The memory circuit 211 is required to be a nonvolatile memory circuit such as a mask ROM, which is capable of holding data even when a power supply is turned off. The memory circuit 211 stores, for example, 16-byte data having 4-byte family code representing the ID chip sequence, 4-byte application code, and two kinds of 4-byte user codes set by users (see FIG. 12A).
The frequency of a transmitted and received signal is 125 kHz, 13.56 MHz, 915 MHz, or 2.45 GHz each having an ISO standard and the like. In addition, modulation and demodulation systems for transmitting and receiving signals are also standardized. An example of such an ID chip is disclosed in Patent Document 1. [Patent Document 1] Japanese Patent Laid-Open No. 2001-250393
The aforementioned conventional semiconductor device as an ID chip has the following problems. If a mask ROM is used as a memory circuit, data cannot be written except during the manufacturing of a chip. Therefore, an ID chip to which data can be written other than during manufacturing is in demand.
If an EEPROM is used as a memory circuit, a user can freely rewrite data but this also allows people other than the original user to rewrite data for identification which should not be rewritten, thereby making forgery possible. Therefore, in order to prevent such forgery, an ID chip to which data can be written only once is in demand.